EXAMINING THE EFFECT OF OLIVINE ADDITION ON OLIVINE LIQUID EQUILIBRIUM TEMPERATURES: IMPLICATIONS FOR THE TEMPERATURES OF THE NORTH ATLANTIC IGNEOUS PROVINCE

Large igneous provinces (LIPs) and seaward dipping reflectors (SDRs) are large outpourings of basaltic magma associated with supercontinent breakup. A hypothesized driver for both the breakup and the volcanism is the impingement of a hot mantle plume at the base of the supercontinent. Plumes ascent is driven by thermal buoyancy, created by the temperature contrast between the rising plume material and the surrounding mantle. The temperature of the mantle source for LIPs and SDRs is determined using relevant thermometers, such as those based on olivine-melt equilibrium. These thermometers are dependent on the calculation of primary magmas, found by reversing the fractional crystallization process until an assumed forsterite content of the residual mantle is attained (Herzberg et al., 2007; Lee et al., 2009; Putirka, 2005).

To examine formation temperature of LIPS and SDRs, we compiled a database of North Atlantic Igneous Province (NAIP) and Central Atlantic Magmatic Province (CAMP) magmas and olivines. From this dataset, we observed a linear trend in NAIP lavas in Ni-MgO space for high Mg number (Mg/(Mg+Fe)) number samples. This linear trend cannot be replicated by fractional crystallization, which predicts rapid depletion of Ni. This suggests that most of the high Mg number magmas in the NAIP are better explained as mixtures of melts with excess olivine. Plotting FeO versus Ni allows us to identify magmas that have escaped olivine addition. These magmas have lower total FeO then previous magmas assumed to be primitive. All other assumptions remaining the same, a lower FeO will result in 60 degree cooler temperatures for the NAIP.

Finally, our compilations show that olivine forsterite contents exhibit a bimodal distribution. We suggest that the high Mg number olivines high nickel values are due to low formation temperatures at high melt degrees. Using the lower Mg number olivines along with the lower FeO contents, we arrive at primary magma temperatures <1425 Celsius. This result suggests that the NAIP and CAMP may not be driven by a thermal plume, implying that supercontinent breakup is driven by far-field forces and LIPs and SDRs are due to lithospheric extension.